1. Field of the Invention
This invention relates to apparatus for changing screen devices which are utilized with plastics extrusion mechanisms and, more particularly, to apparatus which automatically performs screen-changing functions associated with the use of screen devices for filtering extruded plastic materials.
2. Description of the Prior Art
The use of screen devices for filtering extruded plastic materials is well known. Screening is particularly useful when extruding recycled plastics, in order to remove particles of dirt or other foreign matter from the recycled material. Screen devices, however, are particularly subject to clogging up when utilized in plastics recycling operations. Accordingly, the changing of screen devices at frequent intervals is necessary.
Mechanisms for changing screen devices, associated with plastics extruders, are known. For example, U.S. Pat. No. 3,059,276 to L. D. Yokana discloses apparatus wherein each of two parallel paths from an extruder cylinder contains a screen device, and wherein a valving arrangement conducts extruded plastic material alternatively to one or the other of the parallel paths, such that the screen device located in a currently idle path may be changed manually, while the screen device in a currently active path is performing the required filtering operation. In addition, U.S. Pat. No. 3,007,199 to J. E. Curtis discloses apparatus wherein two screen devices are located on a piston, which may be moved between two positions, each locating a different one of the screen devices in line with an extrusion head, such that one screen device may be removed for replacement while the other filters plastic material which is discharged from the extrusion head. The piston of the Curtis patent is power-operated between its two positions. However, each removal of an old screen device from the piston, and each replacement of a new screen device onto the piston, is done manually.
The various screen changing steps required by the prior art mechanisms, as just discussed, involving the manual performance of various functions, are considered unduly time consuming and excessively high in labor costs. This is particularly true in the case of large scale plastics recycling operations, where screen devices must be changed quite frequently, and where the rapid changing of screen devices would be quite advantageous economically.